Acidification of the Mediterranean Sea from anthropogenic carbon penetration

Deep-Sea Research I 102 (2015) 1–15

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Deep-Sea Research I journal homepage: www.elsevier.com/locate/dsri

Acidification of the Mediterranean Sea from anthropogenic carbon penetration Abed El Rahman Hassoun a,b,n, Elissar Gemayel a,b,c, Evangelia Krasakopoulou d, Catherine Goyet b,c, Marie Abboud-Abi Saab a, Véronique Guglielmi b,c, Franck Touratier b,c, Cédric Falco b,c a

National Council for Scientific Research, National Center for Marine Sciences, P.O. Box 534, Batroun, Lebanon IMAGES_ESPACE-DEV, Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 66860 Perpignan Cedex 9, France c ESPACE-DEV, UG UA UR UM IRD, Maison de la télédétection, 500 rue Jean-François Breton, 34093 Montpellier Cedex 5, France d University of the Aegean, Department of Marine Sciences, University Hill, Mytilene 81100, Greece b

art ic l e i nf o

a b s t r a c t

Article history: Received 18 November 2014 Received in revised form 14 April 2015 Accepted 16 April 2015 Available online 29 April 2015

This study presents an estimation of the anthropogenic CO2 (CANT) concentrations and acidification (ΔpH ¼pH2013–pHpre-industrial) in the Mediterranean Sea, based upon hydrographic and carbonate chemistry data collected during the May 2013 MedSeA cruise. The concentrations of CANT were calculated using the composite tracer TrOCA. The CANT distribution shows that the most invaded waters ( 460 mmol kg  1) are those of the intermediate and deep layers in the Alboran, Liguro- and AlgeroProvencal Sub-basins in the Western basin, and in the Adriatic Sub-basin in the Eastern basin. Whereas the areas containing the lowest CANT concentrations are the deep layers of the Eastern basin, especially those of the Ionian Sub-basin, and those of the northern Tyrrhenian Sub-basin in the Western basin. The acidification level in the Mediterranean Sea reflects the excessive increase of atmospheric CO2 and therefore the invasion of the sea by CANT. This acidification varies between  0.055 and  0.156 pH unit and it indicates that all Mediterranean Sea waters are already acidified, especially those of the Western basin where ΔpH is rarely less than  0.1 pH unit. Both CANT concentrations and acidification levels are closely linked to the presence and history of the different water masses in the intermediate and deep layers of the Mediterranean basins. Despite the high acidification levels, both Mediterranean basins are still highly supersaturated in calcium carbonate minerals. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Anthropogenic CO2 Acidification Carbonate system Mediterranean Sea

1. Introduction The Mediterranean Sea is a land-locked relatively small marine ecosystem that represents approximately 0.8% of the world's ocean surface area (EEA, 2002; UNEP/MAP-Plan Bleu, 2009). It is connected to the Atlantic Ocean via the Strait of Gibraltar; it receives surface Atlantic waters (AW) flowing Eastwards and exports intermediate waters to the Atlantic contributing thus to the global overturning circulation (Bergamasco and MalanotteRizzoli, 2010). The Mediterranean Sea is considered a small-scale ocean with high environmental variability and steep physicochemical gradients within a relatively limited region (Béthoux et al., 1999). The circulation is characterized by the presence of subbasin gyres, intense mesoscale activity and a strong seasonal variability related to highly variable atmospheric forcing strongly

n Corresponding author at: National Council for Scientific Research, National Center for Marine Sciences, P.O. Box 534, Batroun, Lebanon. Tel.: þ 961 3 117537. E-mail address: [email protected] (A.E.R. Hassoun).

http://dx.doi.org/10.1016/j.dsr.2015.04.005 0967-0637/& 2015 Elsevier Ltd. All rights reserved.

affected by orographic constraints (Malanotte-Rizzoli et al., 1997; Millot, 1999). The Mediterranean Sea is very special in terms of CΟ2 dynamics, global carbon cycle and anthropogenic CO2 drawdown and storage. Its waters are characterized by high alkalinity ( 2600 mmol kg–1; Schneider et al., 2007; Hassoun et al. 2015b) compared to other oceans. In addition, the Mediterranean waters are slightly more basic (less than a quarter of a pH unit) than the Atlantic waters at corresponding depths (Millero et al., 1979; CARINA project data collection—version 1.2, http:// odv.awi.de/en/data/ocean/carina_bottle_data/). Thus, through the simple acid-base reaction, CO2 is attracted more easily in a slightly more basic seawater than in a slightly less basic seawater. Moreover, its warm and high alkalinity waters, characterized thus by low Revelle factor, are prone to absorb CO2 from the atmosphere and be transported to the interior by the active overturning circulation (Álvarez, 2011). The anthropogenic CO2 inventory for the Mediterranean has been estimated to be 1.7 PgC, thus indicating that this marginal sea has higher CANT concentrations than the global average, mainly determined by the surprisingly high anthropogenic carbon content of the Eastern Mediterranean Sea (Sabine and Tanhua, 2010; Schneider et al., 2010).

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However the role of marginal seas (like the Mediterranean) as a sink of atmospheric CO2 has been understudied because they have been considered to play a minor role in absorbing and storing anthropogenic CO2 due to their small surface area (Lee et al., 2011). Several scientists from various research institutes have recently measured the carbonate system properties (pH; total alkalinity, AT; total dissolved inorganic carbon, CT; and partial pressure of CO2, pCO2) in the Mediterranean Sea. The majority of these studies have been performed in the Western Mediterranean basin (Alekin, 1972; Millero et al., 1979; Copin-Montégut, 1993; Bégovic and Copin-Montégut, 2002; Copin-Montégut and Bégovic, 2002; Copin-Montégut et al., 2004; De Carlo et al., 2013), in the Catalano-Balearic region (Delgado and Estrada, 1994), in the Strait of Gibraltar and the Gulf of Cadiz (Dafner et al., 2001; Santana-Casiano et al., 2002; Huertas et al., 2006; De la Paz et al., 2008; De la Paz et al., 2009; Ribas-Ribas et al., 2011). Whereas fewer ones have been achieved in the other Mediterranean Sea areas: Sicily strait (Chernyakova, 1976); Eastern basin (Schneider et al., 2007; Pujo-Pay et al., 2011). Moreover, few attempts have been dedicated to estimate the CANT penetration in the Mediterranean and the CANT exchanges with the Atlantic (AïtAmeur and Goyet, 2006; Huertas et al., 2009; Flecha et al., 2012). Data attained in the Strait of Gibraltar and the Gulf of Cadiz by the abovementioned studies have indicated that a net export of total inorganic carbon occurs from the Mediterranean to the Atlantic, while there have been contradicting results about the sign of the net CANT that is exchanged between both basins with the exchange being markedly sensitive to the interface definition between the inflowing and the outflowing water bodies and the method considered for CANT estimation. However, the role of the Mediterranean outflowing waters as an important contributor of the North Atlantic CANT inventories has been well recognized (Ríos et al., 2001; Álvarez et al., 2005; Aït-Ameur and Goyet, 2006; Huertas et al., 2009; Flecha et al., 2012). To date, studies devoted to the estimation of the CANT in the Eastern basin are still very scarce (Schneider et al., 2010; Rivaro et al., 2010; Krasakopoulou et al., 2011). In order to quantify the ocean capacity to sequester CANT as well as the increase of acidification, the CANT should be accurately known. Since anthropogenic CO2 cannot be measured directly, as it cannot be chemically discriminated from the bulk of dissolved inorganic carbon, several independent approaches for its indirect estimation have been developed. Using a common and a high quality dataset available for the Atlantic, the Antarctic, and the Arctic Oceans, Vázquez-Rodríguez et al. (2009) performed an inter-comparison exercise of the CANT concentrations estimated by the five most recent models (ΔC*, Gruber et al., 1996; C0IPSL, Lo Monaco et al., 2005; TTD, Waugh et al., 2006; TrOCA, Touratier et al., 2007; and the φC0T, Vázquez-Rodríguez et al., 2009). They concluded that all methods give similar spatial distributions and magnitude of CANT between latitude 601N–401S, and that some differences are found among the methods in the Southern Ocean and the Nordic Seas. The CANT total inventories computed with the TrOCA approach for the whole Atlantic Ocean is 51 PgC; this clearly shows that this approach does not over- or underestimate CANT since it is well in the range of the inventories computed by the four other methods (from 47 to 67 PgC). Several studies have estimated the CANT using the TrOCA approach in the Mediterranean Sea (Touratier and Goyet, 2011), in the Otranto Strait (Krasakopoulou et al., 2011), in the Bay of Biscay (CastañoCarrera et al., 2012) and in the Iberian Sub-basin (Fajar et al., 2012). The differences between the CANT estimations from the φC0T method and those from the TrOCA method are very small (  0.77 74.4 mmol kg  1, n ¼301), and both CANT estimates present the same spatial variations (Castaño-Carrera et al., 2012). Thus, these recent comparisons testify that the TrOCA method provides similar spatial variations as other models and a reasonable upper limit of CANT estimates (within the uncertainty of the results).

Therefore, here this simple and accurate method is chosen to estimate the CANT. Although there are already some quantifications of the CANT in the Mediterranean Sea and some estimations of the acidification evolution in its waters (Touratier and Goyet, 2009; Rivaro et al., 2010; Schneider et al., 2010; Touratier and Goyet 2011; Palmiéri et al., 2015), there is still a lack of data to get a complete picture of the evolution of the CANT and the acidification in this semienclosed sea. Thus, the present study has the following objectives: (1) to quantify the CANT and to examine its trends in the Mediterranean Sea based on the new data of the MedSeA (Mediterranean Sea Acidification In A Changing Climate) cruise collected during May 2013; (2) to characterize the different Mediterranean water masses based on the CANT, and (3) to assess the acidification state of the Mediterranean waters.

1.1. Oceanographic features of the Mediterranean Sea At the Strait of Gibraltar, the AW inflows at the surface layer of the Mediterranean Sea. This water mass flows Eastwards at shallow depth into the Tyrrhenian Sub-basin, then into the Eastern Mediterranean basin via the Strait of Sicily. The salinity of the AW increases along its pathway from 36.5 to4 38 due to net evaporation and is then described as Modified Atlantic Water (MAW; Wüst, 1961). The surface water in the Western Mediterranean basin is supplied by less dense AW through the Strait of Gibraltar (Stöven and Tanhua, 2014). The heat loss of the MAW during winter time along with evaporation leads to a sufficient increase of density to form the Levantine Intermediate Water (LIW) in the Eastern Mediterranean basin (Wüst, 1961; Brasseur et al., 1996). The main volume of the LIW flows back Westwards over the shallow sill between Sicily and Tunisia entering the Tyrrhenian Sub-basin along the continental slope of Italy forming a maximum-salinity layer in a few hundred meters depth (Wüst, 1961). Moreover, the mid-depth waters are also fed by the warm and saline waters (Cretan Intermediate Waters, CIW) formed in the Aegean Sub-basin. These waters outflow through the Western Cretan Straits and circulate in the major part of the intermediate layers of the Ionian Sub-basin. The LIW is a dominant water mass which circulates through both the Eastern and Western basins and is the principal component of the efflux from Gibraltar to the Atlantic Ocean (Roether et al., 1998) with weak contribution of other Mediterranean deep waters [Tyrrhenian Deep Waters (TDW) and Western Mediterranean Deep Waters (WMDW)]. The Mediterranean Sea is a site for deep water mass formation processes. These processes take place in both the Eastern and Western basins; the deep water renewal time has been estimated to be 20–40 years in the Western basin (Stratford et al., 1998) and about 100 years in the Eastern basin (Roether et al., 1996; Stratford and Williams, 1997; Stratford et al., 1998). In the Eastern basin, parts of the LIW enter the Adriatic Sub-basin via the Strait of Otranto, where it serves as an initial source of the EMDWAdr (Stöven and Tanhua, 2014). The formation of EMDWAdr in the Adriatic Sub-basin is based on interactions between the LIW and water masses coming from its northern part as well as the natural preconditioning factors, e.g. wind stress and heat loss (Artegiani et al., 1996a, b; Astraldi et al., 1999; Lascaratos et al., 1999). The EMDWAdr flows then over the sill of Otranto into the Ionian Subbasin intruding the bottom layer and thus representing a main source of the Eastern Mediterranean Deep Water (EMDW) (Schlitzer et al., 1991; Roether and Schlitzer, 1991). However, in the 1990s, the “engine” of the closed thermohaline cell switched to the Aegean Sub-basin (Bergamasco and Malanotte-Rizzoli,

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2010). This latter, which previously had only been a minor contributor to the EMDW, became more effective than the Adriatic Sub-basin as a new source of deep and bottom waters (Eastern Mediterranean Deep Waters originated from the Aegean Subbasin, EMDWAeg) in the Eastern Mediterranean basin (Robinson et al., 2001). In the Western Mediterranean basin, deep water mass formation occurs in two areas: 1) in the Gulf of Lion (the Northwestern part of the Western basin) where the Western Mediterranean Deep Waters (WMDW) are formed and 2) the Tyrrhenian Sub-basin where the Tyrrhenian Deep Waters (TDW) are formed from a mixing between Eastern waters and WMDW. The acronyms of the abovementioned water masses are used thereafter and thus summarized in the Table 1.

2. Methodology 2.1. Study area During the 2013 MedSeA cruise realized on board of the Spanish vessel R/V Angeles Alvariño, from 2 May to 2 June 2013, 23 stations along the Mediterranean Sea were sampled throughout the water column. The major cruise objective is to study, at the basin scale, the impact of elevated CO2 on the Mediterranean Sea biogeochemistry by conducting a comprehensive water column sampling from each of the main Mediterranean basins. The overall goal and scientific objectives of this cruise are further described at the following links: http:// medsea-project.eu/; http://medseaoceancruise.wordpress.com/. The full cruise track (more than 8000 km long) consisted of two almost latitudinal legs. During the first leg, samples were collected from

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Atlantic waters off Cadiz harbor, Spain to the Levantine Sub-basin in the Eastern Mediterranean Sea (3879 km long, 15 stations, 279 sampled points, maximum sampled depth¼ 3720 m). The second leg was conducted in the Northern part of the Mediterranean from the Western Cretan Straits in the Eastern Mediterranean basin to Barcelona, Spain in the North Western Mediterranean basin passing through the South of the Adriatic Sub-basin (3232.5 km long, 8 stations, 183 sampled points, maximum sampled depth¼ 3000 m; Fig. 1).

2.2. Measured properties Hydrographic properties [Salinity, S and Temperature, T (1C)] were measured in situ using a Sea-Bird Electronics CTD system (SBE 911plus) associated with a General Oceanic rosette sampler, equipped with twenty four 12 L Niskin bottles. The precision of measurements is 70.001 1C for T and 70.0003 for S. On board, seawater samples for the dissolved oxygen determination were drawn first from the Niskin bottles followed by the CT/ AT samples taking the precautions recommended to prevent any biological activity and gas exchanges with the atmosphere. Water samples for dissolved oxygen (O2) determination were collected in calibrated BOD 60 mL bottles. Oxygen concentrations were measured using a Winkler iodometric titration (Hansen, 1999) with a Mettler-Toledo DL-21 potentiometric titrator with a Pt ring redox electrode for the determination of the equivalence point (Oudot et al., 1988). The analytical precision and accuracy of O2 measurements are 71.5 mmol kg  1. For AT and CT, seawater samples were collected, at all stations and depths, into pre-washed 500 mL borosilicate glass bottles,

Table 1 The water masses detected in the Mediterranean Sea, their acronyms and the stations numbers used in their characterization in this study. Water masses

Acronyms

Stations

Atlantic Waters Cretan Intermediate Waters Eastern Mediterranean Deep Waters, Adriatic origin Eastern Mediterranean Deep Waters, Aegean origin Levantine Intermediate Waters Levantine Surface Waters Modified Atlantic Waters Tyrrhenian Deep Waters Western Mediterranean Deep Waters

AW CIW EMDWAdr EMDWAeg LIW LSW MAW TDW WMDW

1, 2, 3, 4, 8 10, 14 16, 17 9, 10, 11, 12, 13, 18 9, 10, 11, 12, 13, 14, 17, 19 9, 10, 11, 12, 13 5, 6, 7, 7a 19 20, 21, 22

Fig. 1. Map of the 2013 MedSeA cruise in the Mediterranean Sea. The numbers from 1 to 22 refer to the sampled stations.

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according to standard operational protocol. A small headspace (o1%) was adjusted to prevent pressure build-up and loss of CO2 during storage. The samples were poisoned with a saturated solution of HgCl2 and stored in the dark at constant temperature (  4 1C) until their analysis on shore (at the University of Perpignan, IMAGES, France). The measurement of these two parameters was performed by potentiometric acid titration using a closed cell. The principle and procedure of measurements, as well as a complete description of the system used to perform accurate analysis can be found in the DOE handbook of methods for CO2 analysis (DOE, 1994). The precision of AT and CT analysis was determined to be 7 2 mmol kg  1 for AT and 74 mmol kg  1 for CT, by titrating 261 samples with the same T and S, collected

during May 2013 off Banyuls Sur Mer, South France. Whereas the accuracy of AT and CT measurements was determined to be 71 mmol kg  1 for AT and 74 mmol kg  1 for CT by analyzing a total of 26 bottles of three different batches of Certified Reference Material (CRM, batches 85, 86 and 128, Andrew Dickson, CA, USA) in order to test the sensitivity of the electrodes to the various AT and CT concentrations and the precision of the analysis (Table 2). 2.3. Computed properties In order to estimate accurately the anthropogenic carbon dioxide concentration (CANT) by the TrOCA approach, four parameters were used: potential temperature (θ), dissolved oxygen,

Table 2 AT and CT concentrations ( 7 standard deviation) of the titrated batches of CRM and of the seawater samples. The certified AT and CT concentrations of the 3 batches are also mentioned. Measurements

Number of titrated bottles

Certified AT (lmol kg  1)

Certified CT (lmol kg  1)

Average AT (lmol kg  1)

Average CT (lmol kg  1)

Batch 85 Batch 86 Batch 128 Seawater samples (Banyuls Sur Mer)

5 13 8 261

2184.03 7 0.79 2175.56 7 0.67 2240.287 0.76 –

2000.447 0.43 1998.377 0.54 2013.54 7 0.66 –

2185.79 7 9 2175.9 7 1 22427 4 2565.647 2

1998.617 10 1997.667 4 20147 6 2287.85 7 4

Fig. 2. Distribution of anthropogenic CO2 concentrations (CANT; mmol kg  1) along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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total dissolved inorganic carbon and total alkalinity. Since this approach has been previously described in details (Touratier and Goyet, 2004; Touratier et al., 2007), only the basic equations are presented hereafter. The TrOCA approach is based on the “semi-conservative” tracer TrOCA, which is defined by: TrOCA ¼ O2 þ aðCT  0:5AT Þ

ð1Þ

Similarly to all other approaches, the TrOCA tracer is based upon the Redfield equation and is derived in the same manner as tracers NO and PO defined by Broecker (1974). The conservative tracer TrOCA0 is defined as: TrOCA0 ¼ O02 þ aðC0T  0:5A0T Þ

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The hydrographic and carbonate system data of the 2013 MedSeA cruise are available on Pangaea data repository (Goyet et al., 2015a, 2015b; Ziveri and Grelaud, 2013a, 2013b, 2013c).

3. Results and discussion 3.1. Distribution of CANT concentrations in the Mediterranean Sea Since today, all the various approaches used to estimate CANT in seawater are based upon the Redfield concept which is valid only

ð2Þ

where TrOCA0 is tracer TrOCA but without any anthropogenic contribution and O02 , C0T and A0T are the “pre-industrial” concentrations of O2, CT and AT. By definition, the AT is not affected by the increase of atmospheric CO2, thus A0T ¼ AT. Furthermore, we assume that oxygen is not substantially perturbed by anthropogenic effects (Manning and Keeling, 2006), i.e. O02 EO2. Consequently, the concentration of anthropogenic CO2 (CANT) can be estimated as: CANT ¼

ðTrOCA  TrOCA0 Þ a

ð3Þ

For the estimation of the conservative property TrOCA0, Touratier et al. (2007) established the following Eq. (4) and defined the optimal set of parameters a, b, c and d for the world ocean: TrOCA0 ¼ eðb

þ Cθ þ d=A2T Þ

ð4Þ

The concentration of CANT is then estimated using the four parameters (θ, O2, AT and CT) required to implement the TrOCA approach by applying the following equation via the Ocean Data View [ODV] program (Schlitzer, 2014): CANT ¼

O2 þ 1:279 CT

1  2


2 2 5 AT  eð7:511  ð1:087  10 Þθ  ð7:81  10 =AT ÞÞ 1:279 ð5Þ

In our study, below 300 m, the accuracy of the estimated CANT concentrations using the TrOCA approach is 78.7 μmol kg  1. The pre-industrial CT, used afterwards to calculate the preindustrial pH (pHpre-industrial), was computed by subtracting the 2013 CANT concentrations from the measured 2013CT (Eq. (6)): CT preindustrial ¼ CT 2013 –CANT 2013

ð6Þ

The modern pH (pH2013) was calculated from the modern CT (CT2013) and AT (AT2013). This calculation was fulfilled at the seawater scale, via the “CO2SYS” program configured for Excel by Pierrot et al. (2006), according to the in situ temperature, salinity and pressure conditions, based on AT  CT combination and choosing the set of apparent constants (K1 and K2) of Goyet and Poisson (1989), the sulfate constants of Dickson (1990), and the borate constants of Uppström (1974). Whereas, the pHpre-industrial was estimated from the CTpre-industrial and the 2013 AT since this property is not affected by the accumulation of the CANT in seawater (Touratier and Goyet, 2004). The acidification level (ΔpH) is computed from the difference between the 2013 distribution of pH and the pHpre-industrial: ΔpH ¼ pH2013 –pHpreindustrial

ð7Þ

The saturation degree of calcite (ΩCa) and aragonite (ΩAr) were also computed by using the “CO2SYS” program. The calculation was done according to the same in situ conditions and constants used for the aforementioned computed variables.

Fig. 3. Anthropogenic CO2 concentrations superimposed on θ/S diagrams for the Western basin (a) and the Eastern basin (b). Abbreviations on the figures correspond to the following water masses: AW, Atlantic Water; MAW, Modified Atlantic Water; LSW, Levantine Surface Water; LIW, Levantine Intermediate Water; CIW, Cretan Intermediate Water; EMDW-Aegean, Eastern Mediterranean Deep Water originated from the Aegean Sub-basin; EMDW-Adriatic, Eastern Mediterranean Deep Water originated from the Adriatic Sub-basin; TDW, Tyrrhenian Deep Water; WMDW, Western Mediterranean Deep Water.

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below the wintertime mixed layer depth. None are reliable for the upper ocean where there are biological processes and gas exchanges across the air–sea interface. Therefore, these approaches could not distinguish the CANT from the bulk of CO2 in surface layer. Subsequently, we will exclusively consider the estimations located below the winter mixed layer (4300 m).

3.1.1. General distribution of the CANT In the layers deeper than 300 m down to the bottom, the calculated CANT concentrations vary between 35.2 and 101.9 mmol kg  1 (Fig. 2) and the corresponding mean CANT concentration is equal to 63 mmol kg  1 showing that all the Mediterranean waters are invaded by anthropogenic CO2, in agreement with Touratier and Goyet (2011). The fast overturning circulation in the Mediterranean Sea has led to an overall invasion of the anthropogenic CO2 in the basin. The general profile of CANT shows that its concentration decreases gradually with depth in the Eastern basin to reach its lowest values in the deepest layers (35.55–48 mmol kg  1). Contrariwise, the CANT concentrations are always greater than 70 mmol kg  1 in the Western basin even in the deepest water masses. Rivaro et al. (2010) reported the presence of a slight increase from 300 to 1000 m, and constant values in deep waters of the Mediterranean (29–75 mmol kg  1).

In general, the open ocean CANT concentrations are much lower than those of the Mediterranean Sea. Anthropogenic CO2 generally penetrates to shallower depths in the tropical oceans but to deeper depths in the subtropical ocean (30–401). The symmetry of vertical penetration of anthropogenic CO2 about the equator breaks down toward the poles (latitudes 4501; Lee et al., 2003). In the Western basin of the North Atlantic Ocean between 401N and 501N, the penetration of anthropogenic CO2 based on data collected in 1994, was all the way to the bottom of the water column (Kortzinger et al., 1998), and a CANT mean of 10.4 μmol kg  1 was calculated for the deepest waters. In the Eastern basin of the North Atlantic, the penetration was shallower and CANT concentrations of  5 μmol kg  1 were detected at 3500 m depth. Estimations based on data collected between 1990 and 1998 have shown that in the Northern high-latitude regions, the penetration of anthropogenic CO2, as defined by a 5 μmol kg  1 contour, is greater than 3000 m (Lee et al., 2003). Moderate anthropogenic CO2 concentrations (  10 μmol kg  1) were found below 4000 m between 301S and 501S in the South Atlantic western basin for the year 1994 (Ríos et al., 2010), while previous estimates for that region showed zero and negative values beneath this depth (Lee et al., 2003; Sabine et al., 2004). South of 501S in the South Atlantic, the value of CANT falls sharply to below 5 μmol kg  1 at a depth of only 500 m (Lee et al., 2003). It is thus evident that the atmospheric CO2,

Fig. 4. Distribution of the dissolved oxygen (O2; mmol kg  1) along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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among which the anthropogenic CO2, is efficiently transferred from the atmosphere to the Mediterranean waters, thereby it is well disseminated in this sea, even in the deepest layers. The main reason of the higher uptake and vertical penetration of CO2 in relation to other oceanic regions seems to be the fast deep water formation processes in the Mediterranean Sea combined with surface waters having a relatively low Revelle factor (Schneider et al., 2010), which means that Mediterranean waters are prone to absorb and have the ability to store more CANT than the oceanic ones.

3.1.2. Distribution of the CANT in the Western and Eastern Mediterranean basins Between 300 and 1000 m depth, the CANT varies from 45 to 93 mmol kg  1 in the Western basin and from 37 to 101.9 mmol kg  1 in the Eastern basin. However, it decreases below 1000 m, ranging between 48 and 84 mmol kg  1 and between 35 and 80 mmol kg  1 in the Western and Eastern basins, respectively. These results show that the Western basin of the Mediterranean Sea is more invaded by CANT than the Eastern one, especially in deep layers. Several factors could explain the observed differences: (1) the renewal time of deep waters is considerably different varying between 20 and 40 years in the Western basin (Stratford et al., 1998) and approximately 100 years in the Eastern basin (Roether et al., 1996; Stratford et al., 1998), (2) the high surface temperature and salinity may reduce the solubility and penetration of atmospheric CO2 in the Eastern basin. In this latter, using the Transit Time Distribution (TTD) method, Schneider et al. (2010) estimated the minimum CANT concentration of about 20.5 mmol kg  1 in the intermediate layer which is 15 mmol kg  1 lower than the minimum CANT obtained in the present study using the TrOCA approach. Within a study of the decadal (from the mid-1990s to the mid2000s) evolution of anthropogenic CO2 at the DYFAMED site, in the Northwestern Mediterranean Sea, Touratier and Goyet (2009) reported a decreasing temporal trend of CANT, especially in intermediate and deep layers. They have found a significant correlation between the decrease in CANT and a decrease in the dissolved oxygen that was accompanied by an increase in both salinity and temperature. This latter increase has been related to an enhanced vertical mixing of intermediate waters, coming from the Eastern basin, into the deep waters of the Tyrrhenian Sub-basin, that has

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been noted and associated with the Eastern Mediterranean Transient (EMT) signal (Gasparini et al., 2005; Schröder et al., 2006; Roether and Lupton, 2011). The EMT has played an important role in the intrusion of old water masses characterized by low CANT concentrations, since the waters overflowing in the Sicily Strait have long subsurface travel times from their formation regions (Roether and Lupton, 2011). Using the anthropogenic tracer CFC12, Roether et al. (1998) estimated that the age of the LIW increased by 6 years from 1987 to 1995 as a result of enhanced incorporation of older deep waters into the LIW layer. They also suggested that because of the EMT, the older EMDW of Adriatic origin (low CFC-12 concentration) was uplifted by the newly formed EMDW of Aegean origin (characterized by a higher concentration of CFC-12 since they are much younger). However, the maximum Mediterranean CANT estimation of the current study (101.9 mmol kg  1) is higher than the one earlier provided by Touratier and Goyet (2011; 65 mmol kg  1) after the EMT. The increasing trend of the CANT concentrations between 2001 and 2013 could be attributed to the new deep water masses formation occurred during the intervening period in the Western basin. Actually, Schröder et al. (2008) have confirmed that between 2004 and 2006 almost the whole deep Western basin has been filled with highly saline and warm new deep water, which substantially renewed the resident deep water. On the other hand, the salt content of the LIW, that is an important factor controlling the deep water formation in the Northwestern Mediterranean basin, can explain the high heat and salt contents of the new deep water formed during the severe weather conditions of winter 2004/2005 (Schröder et al., 2008). The newly formed deep waters replaced the old deep ones by uplifting them, which explains the high CANT concentrations in the intermediate and deep layers of the Western sub-basins. In the same direction are also the findings of the CASCADE cruise, conducted during March 2011 in the Northwest part of this basin, that pointed out the key role of the dense shelf waters in the formation of the Western deep waters under determined atmospheric conditions (Durrieu de Madron et al., 2013; Puig et al., 2013). Coastal surface waters over the wide shelf of the Gulf of Lion become denser than the underlying waters and cascade downslope, usually through submarine canyons, until reaching their equilibrium depth (Durieu de Madron et al., 2005). These cold coastal waters which were directly in contact with the atmosphere, thus accumulating high CANT concentrations, are

Table 3 Range (Min., Max.), average and standard deviations (S.D.) of the hydrological and carbonate system parameters in the Mediterranean Sea water masses detected in both the Western and the Eastern basins during the 2013 MedSeA cruise. Parameters

Western basin

Water masses: Number of measurements:

LIW 16

WIW 3

TDW 29

WMDW 44

LIW 18

CIW 8

EMDWAeg 39

EMDWAdr 19

13.38 70.32 13.07–14.13 38.559 7 0.080 38.492–38.733 176.9 7 9.5 164.0–195.4 2588.3 7 20.1 2560–2626 2321.6 7 8.5 2310–2336 66.57 6.0 55.2–73.5 8.048 7 0.026 8.007–8.093 0.1107 0.012 0.087–0.124

13.30 7 0.06 13.22–13.36 38.363 7 0.070 38.266–38.431 202.0 7 19.1 178.2–224.8 2576.3 7 5.0 2571–2583 2317.0 7 11.0 2303–2330 87.17 6.4 78.3–93.2 8.0417 0.015 8.024–8.062 0.142 7 0.008 0.130–0.149

13.18 70.19 12.92–13.75 38.517 70.083 38.166–38.682 186.4 7 6.9 173.2–199.4 2594.7 7 15.2 2572–2619 2310.2 7 9.3 2293–2327 55.8 7 10.3 37.9–74.2 8.059 7 0.028 8.000–8.104 0.090 7 0.018 0.060–0.123

12.92 7 0.03 12.89–13.01 38.4707 0.007 38.458–38.484 197.5 7 1.5 182.8–209.2 2584.0 7 17.9 2555–2629 2314.4 7 11.5 2290–2332 72.6 7 0.3 48.0–89.0 8.016 7 0.032 7.962–8.081 0.1197 0.000 0.078–0.146

14.53 7 0.55 13.80–15.57 38.912 70.087 38.775–39.045 201.17 15.7 174.9–224.4 2613.2 7 26.8 2554–2665 2307.7 7 30.4 2235–2359 69.7 7 13.5 49.6–92.8 8.088 7 0.012 8.061–8.107 0.1067 0.019 0.076–0.136

14.39 7 0.33 13.87–14.78 38.9247 0.091 38.800–39.027 199.9 7 17.2 178.9–220.9 2611.0 7 23.2 2568–2633 2300.7 7 33.6 2259–2330 61.5 7 10.1 49.9–78.3 8.0977 0.018 8.080–8.124 0.093 7 0.016 0.074–0.119

13.62 70.11 13.42–13.88 38.753 7 0.022 38.717–38.804 189.8 7 6.5 177.8–203.8 2608.5 7 8.2 2591–2630 2302.17 12.4 2283–2349 47.4 7 8.9 35.3–80.4 8.067 7 0.028 7.987–8.104 0.0747 0.014 0.055–0.126

13.46 7 0.21 13.04–13.87 38.7327 0.024 38.701–38.789 205.7 7 16.6 175.5–228.1 2619.0 7 11.9 2599–2637 2318.2 7 17.9 2283–2351 67.6 7 20.1 35.9 - 101.9 8.068 7 0.029 7.996–8.102 0.104 7 0.030 0.056–0.156

θ (1C)

Mean 7 S.D. Min.–Max. S Mean 7 S.D. Min.–Max. O2 (lmol kg  1) Mean 7 S.D. Min.– Max. 1 AT (lmol kg ) Mean 7 S.D. Min.– Max. 1 CT (lmol kg ) Mean 7 S.D. Min.– Max. CANT (lmol kg  1) Mean 7 S.D. Min.– Max. pH2013 Mean 7 S.D. Min. – Max. ΔpH Mean 7 S.D. (pH2013  pHpre-ind) Min. – Max.

Eastern basin

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cascading downward to participate in the deep water mass formation. These findings could also explicate the high CANT concentrations recorded in the deep layers of the Western basin. 3.1.3. Distribution of the CANT at sub-basin scale In the Northern part of the Mediterranean Sea, CANT concentrations are higher than 50 mmol kg  1 in the Western basin with relatively low estimates ( 438 mmol kg  1) in the Tyrrhenian Subbasin (Fig. 2). Furthermore, the lowest CANT concentrations were observed in the deep layers of the Ionian Sub-basin (35 mmol kg  1). A study of the evolution of the water mass circulation in the Mediterranean Sea, between 2008 and 2013 (Hassoun et al., 2015a), has proved that the deep layers of the Ionian Sub-basin are still dominated by the EMDW of Aegean origin. This explains the low CANT concentrations noted in this old deep water body where low concentrations of O2 were similarly measured (Fig. 4). In the Tyrrhenian Sub-basin, the relatively low dynamics and the presence of gyre structures are favorable conditions for vertical exchanges (Astraldi and Gasparini, 1994), resulting in the creation of a peculiar water type, the Tyrrhenian Deep Water (TDW), a product of the mixing between LIW and WMDW (Sparnocchia et al., 1999). Based on 3He and tritium data from the period 1987–2009, a downward cascading of inflowing waters from the Eastern basin to the Southern Tyrrhenian Subbasin has been detected, showing a reduced impact of WMDW on

the TDW during the period of enhanced mixing (Roether and Lupton, 2011). Our results are in agreement with Povero et al. (1990) findings, showing that the intermediate and deep waters of this sub-basin have low O2 concentrations (Fig. 4). The previous facts indicate that the Tyrrhenian Sub-basin is filled with old waters, which explains its relatively low CANT concentrations. These results are in a good agreement with the recent findings of Schneider et al. (2014) who, in spite of the detection of a massive input of recently ventilated waters in the Western Mediterranean deep basin, they have found that the ventilation in the Tyrrhenian Sub-basin seemed to be fairly constant since the EMT. The highest CANT concentrations are found in the deep layers of the Adriatic Sub-basin (100 mmol kg  1). This indicates that the waters filling the deep layers of this sub-basin have been recently in contact with the atmosphere as it is confirmed by the relatively high concentrations of O2 (Fig. 4). These results are consistent with the findings of Schneider et al. (2014) who have detected in the Ionian Sub-basin an evidence of increased ventilation after year 2001, indicating the restart of deep water formation in the nearby Adriatic Sub-basin. Moreover, Schneider et al. (2010) hypothesized that waters of the Eastern deep water mass formation sites with high CO2 uptake capacity, which are preconditioned for deep water formation, quickly cool during winter and transfer anthropogenic CO2 into the deep basin.

Fig. 5. Distribution of pH along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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3.1.4. Could we characterize the water masses based on the CANT concentrations? Based on the composite θ/S diagram where the CANT concentrations are superimposed (Fig. 3), we noticed that water masses in the Mediterranean Sea can be easily characterized using the CANT concentrations that differ from one water mass to another (Fig. 3; Table 3). The characterization of the water masses mentioned in Table 3 was realized based on their hydrographic features and the isopycnals. For example, in the Eastern basin, the newly formed EMDW in the Adriatic Sub-basin could be discriminated from the EMDW of Aegean origin by its higher CANT concentrations. Similarly in the Western basin, the WMDW has higher CANT concentrations than those recorded in the relatively old TDW. Although TrOCA approach is not reliable to estimate the CANT concentrations in the upper mixed layer, the surface water masses which are in direct contact with the atmosphere could also be distinguished from the bulk of the Mediterranean intermediate and deep water masses by their extremely high concentrations of CANT (i.e. MAW, LSW). 3.2. Estimation of acidification in the Mediterranean Sea 3.2.1. The pH The average pH of the Mediterranean Sea, during the 2013 MedSeA cruise, is equal to 8.074 70.034. It ranges from a minimum of 7.962 and a maximum of 8.148 (Fig. 5). During May, within

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the upper part of the water column 0–25 m), pH varies between a minimum of 8.049 and a maximum of 8.148. CO2 uptake by phytoplankton increases seawater pH and shifts the dissolved inorganic carbon equilibrium toward carbonate ions. Therefore, the highest pH values have been calculated in the surface layer. The exchange of CO2 with the atmosphere can also cause a change in pH. On a decadal time scale, the accumulation of CANT resulted in a significant reduction of pH compared to the preindustrial era, as mentioned by Touratier and Goyet (2009) from the data of the DYFAMED site. The pH increases Eastward, especially below the surface mixed layer (Fig. 5). In other words, the waters of the Western basin (average pH ¼8.061 70.033) are more acidic than those of the Eastern basin (average pH ¼ 8.08770.024). The lowest pH values were recorded in the intermediate and deep layers of the Western basin. Particularly, they correspond to the layer occupied by the Levantine Intermediate Waters (250 m) in the Alboran Sub-basin, where a zone of minimum oxygen was also detected (Fig. 4), as well as to the deep water mass filling the deepest parts of the Western basin (WMDW). However, the maximum pH values correspond to the Ionian surface water flowing into the Adriatic Sub-basin. 3.2.2. Acidification variations (ΔpH) Despite the knowledge of its potential impacts on biological and chemical processes by the scientific community, estimations

Fig. 6. Acidification (ΔpH) below 300 m, along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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of the acidification in the Mediterranean Sea are still scarce. Variations of the acidification (ΔpH ¼pH2013–pHpre-industrial) in the Mediterranean Sea waters for the two sections of the 2013 MedSeA cruise are presented in Fig. 6. The results show that the Mediterranean Sea water masses are already acidified. A pH decrease ranging between  0.055 and  0.156 pH unit is noted below 300 m. This range is close to the one reported by Touratier and Goyet (2011;  0.05_  0.14 pH unit from pre-industrial period till 2001) and Touratier et al. (2012;  0.061_  0.148 pH unit from pre-industrial period till 2008). As the atmospheric CO2 has increased by 371.13 ppm in 2001 to 385.59 ppm in 2008 and to 396.48 ppm in 2013 (http://www.climate.gov/,http://co2now.org/), the aggravation of the acidification, during the 12 years (between 2001 and 2013), is attributed to the amount of the absorbed anthropogenic CO2 related to the rapid increase of the atmospheric CO2 concentrations emitted by human activities. Moreover, the acidification of the Mediterranean waters is directly associated with its active overturning circulation and the high AT and temperature prevailing throughout the water column. An additional factor could be the fact that only surface anthropogenic CO2loaded waters are inflowing from the Atlantic to the Mediterranean Sea. The ΔpH values of the present study (as well as by Touratier and Goyet, 2011; Touratier et al., 2012) are higher than those mentioned by Palmiéri et al. (2015), although they have the same pattern. This is attributed to the different methods (TrOCA

vs. TTD) used in each study to estimate the anthropogenic CO2 and then to calculate the pre-industrial CT and the pre-industrial pH. Generally, the waters of the Eastern basin are less acidified than those of the Western basin where the ΔpH is always greater than  0.1 pH unit (except in the deep layers of the Adriatic Sub-basin where ΔpH is   0.156 unit; Fig. 6). This finding is related to the difference of the renewal time in each Mediterranean basin, i.e. the renewal time of the Western deep waters is shorter than the Eastern one [20–40 years in the Western basin (Stratford et al., 1998) and about 100 years in the Eastern basin (Roether et al., 1996; Stratford and Williams, 1997; Stratford et al., 1998)]. This fact also explains the higher accumulation of CANT in the Western basin which is more invaded by CANT than the Eastern basin (i.e. low concentrations of CANT in the deep layers of Ionian and Tyrrhenian Sub-basins are highly correlated with the weak levels of acidification, Figs. 2 and 6). 3.3. Impact of the acidification on the formation of biogenic carbonate in the Mediterranean Sea Ocean uptake of anthropogenic CO2 increases the concentration of hydrogen ions (H þ ), thereby it decreases the seawater pH. This reduction in ocean pH has some direct effects on marine organisms (Seibel and Walsh, 2001; Ishimatsu et al., 2005) and decreases carbonate ion concentrations (Cao et al., 2007;

Fig. 7. Degree of calcite saturation along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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Wolf-Gladrow and Rost, 2014), making it more difficult for calcifying marine organisms to form their shells and skeletons (Kroeker et al., 2013 and references therein; Sanford et al., 2014 and references therein; Collins et al., 2014 and references therein; Hilmi et al., 2014 and references therein). In agreement with previous Mediterranean studies (Alekin, 1972; Millero et al., 1979; Schneider et al., 2007; Álvarez et al., 2014), our results show that all Mediterranean waters are strongly saturated with respect to both calcite and aragonite (Ω» 1; Figs.7 and 8). Figs. 7a and 9a indicate that the saturation state of both minerals exhibit a clear longitudinal gradient with increasing values eastward at all depths. The intermediate and deep Mediterranean waters appear more saturated with respect to both calcium carbonate minerals than the Atlantic waters at the same depth. The Eastern basin, which accumulates less CANT, has higher saturation levels for calcite and aragonite (Figs.7 and 9) than those of the Western basin. Based on both sections (the northern and southern ones) conducted along the Mediterranean Sea, it is evident that the upper layers are characterized by higher Ω values than the deeper ones reflecting the effect of pressure on the solubility of the calcium carbonate. Figs. 8 and 10 show the variations of calcite and aragonite (ΩCa2013  ΩCaPre-industrial and ΩAr2013  ΩArPre-industrial), in the

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Mediterranean Sea, between the pre-industrial period and 2013. These figures indicate that their levels decreased in the Mediterranean waters within a range from  0.37 to  2 for ΩCa and from  0.24 to  1.3 for ΩAr. This phenomenon reflects the aggravated effect of the excessive anthropogenic CO2 penetration, and thus of the ocean acidification on the calcium carbonate states, particularly in the sub-basins invaded by high CANT concentrations (i.e. the Western basin and the Adriatic Sub-basin). Time-series studies conducted in the North Pacific Ocean between 1988 and 2007 (Dore et al., 2009) and in the North Atlantic Subtropical gyre between 1983 and 2011 (Bates et al., 2012) have documented a significant long-term decreasing trend of  0.05 pH unit (  0.0019 and 0.0017 unit yr  1 in the North Pacific and the North Atlantic respectively). Comparing the acidification levels in the Mediterranean Sea ranging between  0.055 and  0.156 pH unit with values found in the ocean surface water, this semi-enclosed sea appears as one of the most affected regions by acidification. Notwithstanding, it is still remaining highly supersaturated in both calcite (ΩCa  2.63–6.17; Fig. 7) and aragonite (ΩAr  1.77–4.02; Fig. 8). Therefore, carbonate ions depletion in this area is not an anticipated problem, at least in the near future. However, the high ability of Mediterranean waters to uptake the atmospheric CO2 which is then rapidly transported to

Fig. 8. The variation of calcite saturation between 2013 and the pre-industrial period along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

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Fig. 9. Degree of aragonite saturation along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

the sea interior with the overturning circulation, has the potential to alter the carbonate saturation conditions, with consequences not yet clearly defined for the well adapted organisms. In addition, high acidification levels can influence the microbial nutrient cycling and the speciation of nutrients in the Mediterranean Sea. There is a high probability that the decreasing pH may increase its oligotrophic nature and the degree of phosphorus limitation by changing the N:P stoichiometry due to reductions in phosphate concentrations and shifts in the nitrogen speciation (CIESM, 2008 and references therein). During a mesocosm experiment off Corsica performed in the frame of MedSeA project, the nitrogen fixation rates were seen to increase approximately 10-fold for pH levels predicted after  2100 (pH o7.73) (Rees et al., 2013), implying future changes of the nitrogen cycle within the Mediterranean with likely consequences on ecosystem functioning due to changes in community structure.

4. Conclusions In this paper, the distributions of anthropogenic CO2, pH2013 and acidification (between pre-industrial period and 2013) in the Mediterranean Sea, have been presented and discussed. All Mediterranean waters are invaded by anthropogenic CO2 with

concentrations much higher than those recorded in other oceanic areas. This fact is attributed to the effective atmospheric CO2 absorption at the surface and to the short deep water renewal time in this sea. In general, the Eastern basin accumulates less CANT than the Western basin. Moreover, the most invaded waters by CANT (460 mmol kg  1) have been detected in the intermediate (300–500 m) and deep (4 500 m) layers:1- of the Alboran, Liguro and Algero-Provencal Sub-basins in the Western basin, and 2- of the Adriatic Sub-basin in the Eastern basin. This fact shows that these waters have recently been in contact with the atmosphere. Whereas the areas containing the lowest CANT concentrations are the deep layers of the Eastern basin, especially those of the Ionian Sub-basin and those of the northern Tyrrhenian Sub-basin in the Western basin where the concomitant low O2 concentrations indicate that the prevailing waters are very old. The acidification (ΔpH between 2013 and the pre-industrial period) of the Mediterranean Sea reflects the excessive increase of atmospheric CO2 and therefore the invasion of the sea by CANT. This acidification ranges from  0.055 to 0.156 pH unit. It indicates that all Mediterranean Sea waters are already acidified, especially those of the Western basin where ΔpH is rarely less than  0.1 pH unit. Although both basins are supersaturated with calcite and aragonite, respectively, the Western basin is characterized by

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Fig. 10. The variation of aragonite saturation between 2013 and the pre-industrial period along (a) the southern and (b) the northern sections of the 2013 MedSeA cruise.

carbonate ions concentrations and saturation degrees lower than those in the other basin. This shows that, at a large time scale, the pH decrease can influence the dissolution of carbonate ions and then the biological activity of several marine organisms, especially the calcifying ones.

Acknowledgment This work was funded by the EC FP7 “Mediterranean Sea Acidification in a changing climate” project (MedSeA; grant agreement 265103; medsea-project.eu). The authors are pleased to thank the captain, the crew of the Spanish research vessel R/V Ángeles Alvariño and the chief scientists of the 2013 MedSeAcruise: Patrizia Ziveri (the coordinator of the MedSeA project) and Jordi Garcia-Orellana, for their excellent cooperation during and after the campaign. Moreover, the authors would like to thank the colleagues, Michele Giani, Gianmarco Ingrosso and Mauro Celussi, from OGS/Italy, for their contribution in the oxygen analysis. Authors are also grateful to the National Council for Scientific Research (CNRS) in Lebanon for the PhD thesis scholarships granted to Abed El Rahman Hassoun and to Elissar Gemayel.

References Aït-Ameur, N., Goyet, C., 2006. Distribution and transport of natural and anthropogenic CO2 in the Gulf of Cadiz. Deep-Sea Res. II: Top. Stud. Oceanogr. 53 (1113), 1329–1343. Alekin, O.A., 1972. Saturation of Mediterranean Sea water with calcium carbonate. Geochemistry 206, 239–242. Álvarez, M., Perez, F.F., Shoosmith, D.R., Bryden, H.L., 2005. Unaccounted role of Mediterranean Water in the drawdown of anthropogenic carbon. J. Geophys. Res. 110, C09S03. http://dx.doi.org/10.1029/2004JC002633. Álvarez, M., 2011. The CO2 system observations in the Mediterranean sea: past, present and future. In: Briand, F. (Ed.), Designing Med-SHIP: A Program for Repeated Oceanographic Surveys. No. 43 in CIESM Workshop Monographs, Monaco, 164. Álvarez, M., Sanleón-Bartolomé, H., Tanhua, T., Mintrop, L., Luchetta, A., Cantoni, C., Schroeder, K., Civitarese, G., 2014. The CO2 system in the Mediterranean Sea: a basin wide perspective. Ocean Sci. 10, 69–92. Artegiani, A., Bregant, D., Paschini, E., Pinardi, N., Raicich, F., Russo, A., 1996a. The Adriatic Sea general circulation, Part I: Air–sea interactions and water mass structure. J. Phys. Oceanogr. 27, 1492–1514. Artegiani, A., Bregant, D., Paschini, E., Pinardi, N., Raicich, F., Russo, A., 1996b. The Adriatic Sea General Circulation, Part II: Baroclinic circulation structure. J. Phys. Oceanogr. 27, 1515–1532. Astraldi, M., Gasparini, G.P., 1994. The seasonal characteristics of the circulation in the Tyrrhenian Sea. In: La Violette, P.E. (Ed.), Seasonal and Interannual Variability of the Western Mediterranean Sea. American Geophysical Union, pp. 115–134. Astraldi, M., Balopoulos, S., Candela, J., Font, J., Gacic, M., Gasparini, G.P., Manca, B., Theocharis, J., Tintoré, J., 1999. The role of straits and channels in understanding

14

A.E.R. Hassoun et al. / Deep-Sea Research I 102 (2015) 1–15

the characteristics of Mediterranean circulation. Prog. Oceanogr. 44 (1–3), 65–108. Bates, N.R., Best, M.H.P., Neely, K., Garley, R., Dickson, A.G., Johnson, R.J., 2012. Detecting anthropogenic carbon dioxide uptake and ocean acidification in the North Atlantic Ocean. Biogeosciences 9, 2509–2522. http://dx.doi.org/10.5194/ bg-9-2509-2012. Bégovic, M., Copin-Montégut, C., 2002. Processes controlling annual variations in the partial pressure of CO2 in surface waters of the central northwestern Mediterranean Sea (Dyfamed site). Deep-Sea Res. II: Top. Stud. Oceanogr. 49 (11), 2031–2047. Bergamasco, A., Malanotte-Rizzoli, P., 2010. The circulation of the Mediterranean Sea: a historical review of experimental investigations. Adv. Oceanogr. Limnol. 1 (1), 11–28. Béthoux, J.P., Gentili, B., Morin, P., Nicolas, E., Pierre, C., Ruiz-Pino, D., 1999. The Mediterranean Sea: a miniature ocean for climatic and environmental studies and a key for the climatic functioning of the North Atlantic. Prog. Oceanogr. 44 (1–3), 131–146. Brasseur, P., Beckers, J.M., Brankart, J.M., Schoenauen, R., 1996. Seasonal temperature and salinity fields in the Mediterranean Sea: climatological analyses of an historical data set. Deep-Sea Res. I: Oceanogr. Res. Pap. 43 (2), 159–192. Broecker, W.S., 1974. “NO”, a conservative water-mass tracer. Earth Planet. Sci. Lett. 23 (1), 100–107. Cao, L., Caldeira, K., Jain, A.K., 2007. Effects of carbon dioxide and climate change on ocean acidification and carbonate mineral saturation. Geophys. Res. Lett. 34 (L05607), http://dx.doi.org/10.1029/2006GL028605. Castaño-Carrera, M., Pardo, P.C., Alvarez, M., Lavín, A., Rodríguez, C., Carballo, R., Ríos, A.F., Pérez, F.F., 2012. Anthropogenic carbon and water masses in the Bay of Biscay. Cienc. Mar. 38 (1B), 191–207. Chernyakova, A.M., 1976. Elements of the carbonate system in the Straits of Sicily (Tunis Strait) area. Oceanology 16, 36–39. CIESM, 2008. Impacts of acidification on biological, chemical and physical systems in the Mediterranean and Black Seas. In: Briand, F. (Ed.), No. 36 in CIESM Workshop MonographsMonaco, p. 124. Collins, S., Rost, B., Rynearson, T.A., 2014. Evolutionary potential of marine phytoplankton under ocean acidification. Evol. Appl. 7, 140–155. http://dx.doi. org/10.1111/eva.12120. Copin-Montégut, C., 1993. Alkalinity and carbon budgets in the Mediterranean Sea. Glob. Biogeochem. Cycles 7 (4), 915–925. http://dx.doi.org/10.1029/93GB01826. Copin-Montégut, C., Bégovic, M., 2002. Distributions of carbonate properties and oxygen along the water column (0–2000 m) in the central part of the NW Mediterranean Sea (Dyfamed site). Influence of the winter vertical mixing on air-sea CO2 and O2 exchanges. Deep-Sea Res. II: Top. Stud. Oceanogr. 49 (11), 2049–2066. Copin-Montégut, C., Bernard, A., Claire, C., 2004. Continuous pCO2 measurements in surface water during transect EUM4LPEU , http://dx.doi.org/10.1594/ PANGAEA.139915. Dafner, E., González Dávila, M., Santana Casiano, J.M., Semperé, R., 2001. Total organic and inorganic carbon exchange through Strait of Gibraltar in September 97. Deep-Sea Res. I: Oceanogr. Res. Pap. 48, 1217–1235. De Carlo, E.H., Mousseau, L., Passafiume, O., Drupp, P.S., Gattuso, J.-P., 2013. Carbonate chemistry and air–sea CO2 flux in a NW Mediterranean Bay over a four-year period: 2007–2011. Aquat. Geochem. 19, 399–442. http://dx.doi.org/ 10.1007/s10498-013-9217-4. De la Paz, M., Debelius, B., Macias, D., Gomez-Parra, A., Forja, J., 2008. Inorganic carbon dynamic and the influence of tidal mixing processes on the Strait of Gibraltar (SW Spain). Cont. Shelf Res. 38, 1827–1837. De la Paz, M., Gomez-Parra, A., Forja, J., 2009. Seasonal variability of surface fCO2 in the Strait of Gibraltar. Aquat. Sci. 71, 55–64. Delgado, O., Estrada, M., 1994. CO2 system in a Mediterranean frontal zone. Sci. Mar. 58 (3), 237–250. Dickson, A.G., 1990. Standard potential of the reaction: AgCl(s) þ 1/2H2 ¼ Ag(s) þHCl (aq), and the standard acidity constant of the ion HSO4 in synthetic sea water from 273.15 to 318.15 K. J. Chem. Thermodyn. 22, 113–127. DOE, 1994. Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. Version 2. In: Dickson, A.G. Goyet, C. (Eds.), ORNL/CDIAC-74. Dore, J.E., Lukas, R., Sadler, D.W., Church, M.J., Karl, D.M., 2009. Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Proc. Natl. Acad. Sci. USA 106 (30), 12235–12240. http://dx.doi.org/10.1073/ pnas.0906044106. Durieu de Madron, X., Zervakis, V., Theocharis, A., Georgopoulos, D., 2005. Comments on “Cascades of dense water around the world”. Prog. Oceanogr. 64, 83–90. Durrieu de Madron, X., Houpert, L., Puig, P., Sanchez-Vidal, A., Testor, P., Bosse, A., Estournel, C., Somot, S., Bourrin, F., Bouin, M.N, Beauverger, M., Beguery, L., Calafat, A., Canals, M., Coppola, L., Dausse, D., D'Ortenzio, F., Font, J., Heussner, S., Kunesch, S., Lefevre, D., Le Goff, H., Martín, J., Mortier, L., Palanques, A., Raimbault, P., 2013. Interaction of dense dense shelf water cascading and open-sea convection in the Northwestern Mediterranean during winter 2012. Geophys. Res. Lett. 40, 1379–1385. http://dx.doi.org/10.1002/grl.50331. EEA, 2002. Europe's biodiversity—biogeographical regions and seas. Seas around Europe the Mediterranean Sea—blue oxygen-rich, nutrient poor waters.. Fajar, N.M., Pardo, P.C., Carracedo, L., Vázquez-Rodríguez, M., Ríos, A.F., Pérez, F.F., 2012. Trends of the anthropogenic CO2 along 201W in the Iberian Basin. Cienc. Mar. 38 (1B), 287–306.

Flecha, S., Pérez, F.F., Navarro, G., Ruiz, J., Olivé, I., Rodríguez-Gálvez, S., Costas, E., Huertas, I.E., 2012. Anthropogenic carbon inventory in the Gulf of Cádiz. J. Mar. Syst. 92, 67–75. Gasparini, G.P., Ortona, A., Budillon, B., Astraldi, M., Sansone, E., 2005. The effect of the Eastern Mediterranean Transient on the hydrographic characteristics in the Strait of Sicily and in the Tyrrhenian Sea. Deep-Sea Res. I: Oceanogr. Res. Pap. 52, 915–935. Goyet, C., Poisson, A., 1989. New determination of carbonic-acid dissociationconstants in seawater as a function of temperature and salinity. Deep-Sea Res. Part A. Oceanogr. Res. Pap. 36 (11), 1635–1654. Goyet, C., Hassoun, A.E.R., Gemayel, E., 2015a. Carbonate system during the May 2013 MedSeA cruise , http://dx.doi.org/10.1594/PANGAEA.841933. Goyet, C., Gemayel, E., Hassoun, A.E.R., 2015b. CO2Underway pCO2 in surface water during the 2013 MedSEA cruise , http://dx.doi.org/10.1594/PANGAEA.841928. Gruber, N., Sarmiento, J.L., Stocker, T.F., 1996. An improved method for detecting anthropogenic CO2 in the oceans. Glob. Biogeochem. Cycles 10, 809–837, doi:199610.1029/96GB01608. Hansen, H.P., 1999. Determination of oxygen. Methods of seawater analysis. In: Grasshoff, M., Kremling, K., Ehrhardt, M. (Eds.), VerlagChemie, Weinheim, pp. 75–89. Hassoun, A.E.R., Guglielmi, V., Gemayel, E., Goyet, C., Abboud-Abi Saab, M., Giani, M., Ziveri, P., Ingrosso, G., Touratier, F., 2015a. Is the Mediterranean Sea circulation in a steady state. J. Water Resour. Ocean Sci. 4 (1), 6–17. http://dx.doi.org/10.11648/j. wros.20150401.12. Hassoun, A.E.R., Gemayel, E., Krasakopoulou, E., Goyet, C., Abboud-Abi Saab, M., Ziveri, P., Touratier, F., Guglielmi, V., Falco, C., 2015b. Modeling of the total alkalinity and the total inorganic carbon in the Mediterranean Sea. J. Water Resour. Ocean Sci. 4 (1), 24–32. http://dx.doi.org/10.11648/j.wros.20150401.14. Hilmi, N., Allemand, D., Haraldsson, G., Reynaud, S., Cinar, M., Jeffree, R.A., Cooley, S., Hattam, C., Safa, A., Dupont, S., 2014. Exposure of Mediterranean countries to ocean acidification. Water 6, 1719–1744. http://dx.doi.org/10.3390/w6061719. Huertas, I.E., Navarro, G., Rodriguez-Galvez, S., Lubian, L.M., 2006. Temporal patterns of carbon dioxide in relation to hydrological conditions and primary production in the northeastern shelf of the Gulf of Cadiz (SW Spain). Deep-Sea Res. II: Top. Stud. Oceanogr. 53, 1344–1362. Huertas, I.E., Rios, A.F., Garcia-Lafuente, J., Makaoui, A., Rodriguez-Galvez, S., Sanchez-Roman, A., Orbi, A., Ruiz, J., Pérez, F.F., 2009. Anthropogenic and natural CO2 exchange through the Strait of Gibraltar. Biogeosciences 6, 647–662. Ishimatsu, A., Hayashi, M., Lee, K.S., Kikkawa, T., Kita, J., 2005. Physiological effects of fishes in a high-CO2 world. J. Geophys. Res. 110 (10) 1029/2004JC002564. Kortzinger, A., Mintrop, L., Duinker, J.C., 1998. On the penetration of anthropogenic CO2 into the North Atlantic Ocean. J. Geophys. Res. 103 (C9), 18681–18689. Krasakopoulou, E., Souvermezoglou, E., Goyet, C., 2011. Anthropogenic CO2 fluxes in the Otranto Strait (E. Mediterranean) in February 1995. Deep-Sea Res. I: Oceanogr. Res. Pap. 58, 1103–1114. Kroeker, K.J., Kordas, R.L., Crim, R., Hendriks, I.E., Ramajo, L., Singh, G.S., Duarte, C.M., Gattuso, J.-P., 2013. Impacts of ocean acidification on marine organisms: quantifying sensitivities and interaction with warming. Glob. Change Biol. 19, 1884–1896. http://dx.doi.org/10.1111/gcb.12179. Lascaratos, A., Roether, W., Nittis, K., Klein, B., 1999. Recent changes in deep water formation and spreading in the eastern Mediterranean Sea: a review. Prog. Oceanogr. 44, 5–36. Lee, K., Choi, S.D., Park, G.H., Wanninkhof, R., Peng, T.H., Key, R.M., Sabine, C.L., Feely, R.A., Bullister, J.L., Millero, F.J., 2003. An updated anthropogenic CO2 inventory in the Atlantic Ocean. Glob. Biogeochem. Cycles 17 (4), 1116. http: //dx.doi.org/10.1029/2003GB002067. Lee, K., Sabine, C.L., Tanhua, T., Kim, T.W., Feely, R.A., Kim, H.C., 2011. Roles of marginal seas in absorbing and storing fossil fuel CO2. Energy Environ. Sci. 4, 1133–1146. http://dx.doi.org/10.1039/C0EE00663G. Lo Monaco, C., Goyet, C., Metzl, N., Poisson, A., Touratier, F., 2005. Distribution and inventory of anthropogenic CO2 in the Southern Ocean: comparison of three data-based methods. J. Geophys. Res. 110, C09S02. http://dx.doi.org/10.1029/ 2004JC002571. Malanotte-Rizzoli, P., Manca, B.B., Ribera d'Alcalà, M., Theocharis, A., Bergamasco, A., Bregant, D., Budillon, G., Civitarese, G., Georgopoulos, D., Michelato, A., Sansone, E., Scarazzato, P., Souvermezoglou, E., 1997. A synthesis of the Ionian Sea hydrography, circulation and water mass pathways during POEM-Phase I. Prog. Oceanogr. 39, 153–204. Manning, A.C., Keeling, R.F., 2006. Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network. Tellus 58B, 95–116. Millero, F.J., Morse, J., Chen, C.T., 1979. The carbonate system in the western Mediterranean sea. Deep-Sea Res. A Oceanogr. Res. Pap. 26 (12), 1395–1404. Millot, C., 1999. Circulation in the Western Mediterranean Sea. J. Mar. Syst. 20 (1–4), 423–442. Oudot, C., Gerard, R., Morin, P., Gningue, I., 1988. Precise shipboard determination of dissolved oxygen (Winkler procedure) for productivity studies with commercial system. Limnol. Oceanogr. 33, 146–150. Palmiéri, J., Orr, J.C., Dutay, J.C., Béranger, K., Schneider, A., Beuvier, J., Somot, S., 2015. Simulated anthropogenic CO2 uptake and acidification of the Mediterranean Sea. Biogeosciences 12, 781–802. http://dx.doi.org/10.5194/bg-12-7812015. Pierrot, D., Lewis, E., Wallace, D.W.R., 2006. MS Excel Program Developed for CO2 System Calculations, ORNL/CDIAC-105. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee.

A.E.R. Hassoun et al. / Deep-Sea Research I 102 (2015) 1–15

Povero, P., Hopkins, T.S., Fabiano, M., 1990. Oxygen and nutrient observations in the Southern Tyrrhenian Sea. Oceanol. Acta 13 (3), 299–305. Puig, P., Durrieu de, Madron X., Salat, J., Schroeder, K., Martín, J., Karageorgis, A.P., Palanques, A., Roullier, F., Lopez-Jurado, J.L., Emelianov, M., Moutin, T., Houpert, L., 2013. Thick bottom nepheloid layers in the western Mediterranean generated by deep dense shelf water cascading. Prog. Oceanogr. 111, 1–23. Pujo-Pay, M., Conan, P., Oriol, L., Cornet-Barthaux, V., Falco, C., Ghiglione, J.F., Goyet, C., Moutin, T., Prieur, L., 2011. Integrated survey of elemental stoichiometry (C, N, P) from the western to eastern Mediterranean Sea. Biogeosciences 8, 883–899. http: //dx.doi.org/10.5194/bg-8-883-2011. Rees A.P., Clark D.R., Turk-Kubo K.A., Zehr J.P., and Al-Moosawi L., 2013. Acidification of the marine nitrogen cycle. In: ASLO 2013 Aquatic Sciences Meeting, 17–22 February 2013, New Orleans, Louisiana, USA. 〈http://www.sgmeet.com/aslo/ neworleans2013/viewabstract2.asp?AbstractID=10910〉. Ribas-Ribas, M., Gómez-Parra, A., Forja, J.M., 2011. Air–sea CO2 fluxes in the northeastern shelf of the Gulf of Cádiz (southwest Iberian Peninsula). Mar. Chem. 123, 56–66. Ríos, A.F., Pérez, F.F., Fraga, F., 2001. Long-term (1977–1997) measurements of carbon dioxide in the Eastern North Atlantic: evaluation of anthropogenic input. Deep-Sea Res. II: Top. Stud. Oceanogr. 48, 2227–2239. Ríos, A.F., Vázquez-Rodríguez, M., Padín, X.A., Pérez, F.F., 2010. Anthropogenic carbon dioxide in the South Atlantic western basin. J. Mar. Syst. 83, 38–44. Rivaro, P., Messa, R., Massolo, S., Frache, R., 2010. Distributions of carbonate properties along the water column in the Mediterranean Sea: spatial and temporal variations. Mar. Chem. 121, 236–245. Robinson, A.R., Leslie, W.G., Theocharis, A., Lascaratos, A., 2001. Mediterranean Sea circulation. In: Steele, J.H., et al. (Eds.), Encyclopedia of Ocean Sciences. Elsevier, New York, pp. 1689–1705. Roether, W., Schlitzer, R., 1991. Eastern Mediterranean deep water renewal on the basis of chlorofluoromethane and tritium data. Dyn. Atmos. Ocean. 15, 333–354. Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevic, V., Luchetta, A., 1996. Recent changes in Eastern Mediterranean deep waters. Science 271, 333–335. Roether, W., Klein, B., Beitzel, V., Manca, B.B., 1998. Property distributions and transient-tracer ages in Levantine Intermediate Water in the Eastern Mediterranean. J. Mar. Syst. 18, 71–87. Roether, W., Lupton, J.E., 2011. Tracers confirm downward mixing of Tyrrhenian Sea upper waters associated with the Eastern Mediterranean Transient. Ocean Sci. 7, 91–99. http://dx.doi.org/10.5194/os-7-91-2011. Sabine, C.L., Feely, R.A., Gruber, N., Key, R.M., Lee, K., Bullister, J.L., Wanninkhof, R., Wong, C.S., Wallace, D.W.R., Tilbrook, B., Millero, F.J., Peng, T.-H., Kozyr, A., Ono, T., Ríos, A.F., 2004. The oceanic sink for anthropogenic CO2. Science 305, 367–371. Sabine, C.L., Tanhua, T., 2010. Estimation of anthropogenic CO2 inventories in the ocean. Annu. Rev. Mar. Sci. 2, 175–198. Sanford, E., Gaylord, B., Hettinger, A., Lenz, E.A., Meyer, K., Hill, T.M., 2014. Ocean Acidification Increases the Vulnerability of Native Oysters to Predation by Invasive Snails. Royal Society Publishinghttp://dx.doi.org/10.1098/rspb.2013.2681. Santana-Casiano, J.M., Gonzalez-Davila, M., Laglera, L.M., 2002. The carbon dioxide system in the Strait of Gibraltar. Deep-Sea Res. II: Top. Stud. Oceanogr. 49, 4145–4161. Schlitzer, R., Roether, W., Oster, H., Junghans, H., Hausmann, M., Johannsen, H., Michelato, A., 1991. Chlorofluoromethane and oxygen in the eastern Mediterranean. Deep-Sea Res. I: Oceanogr. Res. Pap. 38, 1531–1551. Schlitzer R., 2014. Ocean Data View, 〈http://odv.awi.de〉. Schröder, K., Gasparini, G.P., Tangherini, M., Astraldi, M., 2006. Deep and intermediate water in the western Mediterranean under the influence of the Eastern Mediterranean Transient. Geophys. Res. Lett. 33 (L21607), http://dx.doi.org/ 10.1029/2006GL027121. Schröder, A., Ribotti, A., Borghini, M., Sorgente, R., Perilli, A., Gasparini, G.P., 2008. An extensive western Mediterranean deep water renewal between 2004 and 2006. Geophys. Res. Lett. 35 (L18605), http://dx.doi.org/10.1029/2008GL035146. Schneider, A., Wallace, D.W.R., Körtzinger, A., 2007. Alkalinity of the Mediterranean Sea. Geophys. Res. Lett. 34, L15608. http://dx.doi.org/10.1029/2006GL028842. Schneider, A., Tanhua, T., Körtzinger, A., Wallace, D.W.R., 2010. High anthropogenic carbon content in the eastern Mediterranean. J. Geophys. Res. 115, C12050. http://dx.doi.org/10.1029/2010JC006171.

15

Schneider, A., Tanhua, T., Roether, W., Steinfeldt, R., 2014. Changes in ventilation of the Mediterranean Sea during the past 25 year. Ocean Sci. 10, 1–16. http://dx. doi.org/10.5194/os-10-1-2014. Seibel, B.A., Walsh, P.J., 2001. Potential impacts of CO2 injection on deep-sea biota. Science 294, 319–320. Sparnocchia, S., Gasparini, G.P., Astraldi, M., Borghini, M., Pistek, P., 1999. Dynamics and mixing of the Eastern Mediterranean outflow in the Tyrrhenian Sea. J. Mar. Syst. 20, 301–317. Stöven, T., Tanhua, T., 2014. Ventilation of the Mediterranean Sea constrained by multiple transient tracer measurements. Ocean Sci. 10, 439–457. http://dx.doi. org/10.5194/os-10-439-2014. Stratford, K., Williams, R., 1997. A tracer study of the formation, dispersal, and renewal of Levantine Intermediate Water. J. Geophys. Res. 102 (C6), 12539–12549. Stratford, K., Williams, R.G., Drakopoulos, P.G., 1998. Estimating climatological age from a model-derived oxygen-age relationship in the Mediterranean. J. Mar. Syst. 18, 215–226. Touratier, F., Goyet, C., 2004. Applying the new TrOCA approach to assess the distribution of anthropogenic CO2 in the Atlantic Ocean. J. Mar. Syst. 46, 181–197. Touratier, F., Goyet, C., Azouzi, L., 2007. CFC-11, 114C, and 3H tracers as a means to assess anthropogenic CO2 concentrations in the ocean. Tellus 59B, 318–325. Touratier, F, Goyet, C., 2009. Decadal evolution of anthropogenic CO2 in the northwestern Mediterranean Sea from the mid-1990s to the mid-2000s. Deep-Sea Res. I: Oceanogr. Res. Pap. 156, 1708–1716. Touratier, F., Goyet, C., 2011. Impact of the Eastern Mediterranean Transient on the distribution of anthropogenic CO2 and first estimate of acidification for the Mediterranean Sea. Deep-Sea Res. I: Oceanogr. Res. Pap. 58, 1–15. Touratier, F., Guglielmi, V., Goyet, C., Prieur, L., Pujo-Pay, M., Conan, P., Falco, C., 2012. Distributions of the carbonate system properties, anthropogenic CO2, and acidification during the 2008 BOUM cruisem (Mediterranean Sea). Biogeosci. Discussion 9, 2709–2753. http://dx.doi.org/10.5194/bgd-9-2709-2012. UNEP/MAP-Plan Bleu, 2009. State of the Environment and Development in the Mediterranean. UNEP/MAP-Blue Plan, Athens p. 200. Uppström, L.R., 1974. The boron/chlorinity ratio of the deep-sea water from the Pacific Ocean. Deep-Sea Res. Oceanogr. Abstr. 21 (2), 161–162. Vázquez-Rodríguez, M., Touratier, F., Lo Monaco, C., Waugh, D.W., Padin, X.A., Bellerby, R.G.J., Goyet, C., Metzl, N., Ríos, A.F., Pérez, F.F., 2009. Anthropogenic carbon distributions in the Atlantic Ocean: data-based estimates from the Arctic to the Antarctic. Biogeosciences 6, 439–451. http://dx.doi.org/10.5194/ bg-6-439-2009. Waugh, D.W., Hall, T.M., McNeil, B.I., Key, R., Matear, R.J., 2006. Anthropogenic CO2 in the oceans estimated using transit time distributions. Tellus B 58, 376–389. http://dx.doi.org/10.1111/j.1600-10.0889.2006.00222.x. Wolf-Gladrow, D.A., Rost, B., 2014. Ocean Acidification and Oceanic Carbon Cycling. Global Environmental Change. Handbook of Global Environmental Pollution, vol. 1, pp. 103–110. Wüst, G., 1961. On the vertical circulation of the Mediterranean Sea. J. Geophys. Res. 66 (10), 3261–3271. http://dx.doi.org/10.1029/JZ066i010p03261. Ziveri, P., Grelaud, M., 2013a. Continuous thermosalinograph oceanography along Ángeles Alvariño cruise track MedSeA2013. Universitat Autònoma de Barcelona , http://dx.doi.org/10.1594/PANGAEA.822153. Ziveri, P., Grelaud, M., 2013b. Physical oceanography during Ángeles Alvariño cruise MedSeA2013. Universitat Autònoma de Barcelona , http://dx.doi.org/10.1594/ PANGAEA.822162. Ziveri, P., Grelaud, M., 2013c. Physical oceanography measured on water bottle samples during Ángeles Alvariño cruise MedSeA2013. Universitat Autònoma de Barcelona , http://dx.doi.org/10.1594/PANGAEA.822163. 〈http://odv.awi.de/en/data/ocean/carina_bottle_data/〉, CARINA project data collection—version 1.2. 〈http://www.climate.gov/〉. 〈http://co2now.org/〉. 〈http://medsea-project.eu/〉/. 〈http://medseaoceancruise.wordpress.com/〉.